1. Field of the Invention
The present invention relates to a semiconductor device and a driving method thereof. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. A transistor can be given as an example of such a semiconductor element. Therefore, the semiconductor device includes a liquid crystal display device, a memory device, and the like in its category.
2. Description of the Related Art
As a kind of volatile memory, a static random access memory (SRAM) is widely known. For example, disclosed in Patent Document 1 is a semiconductor integrated circuit in which data in an SRAM is saved to an external context storage unit, so that power consumption and start-up time can be reduced.
Meanwhile, in recent years, metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) have attracted attention. Oxide semiconductors can be applied to transistors (see Patent Documents 2 and 3).